Lecithin: cholesterol acyl transferase (LCAT) is a key factor in plasma cholesterol metabolism. The enzyme consequently may have a role in lowering plasma cholesterol levels or maintaining the shape of lipoproteins. In either case LCAT is likely to be involved in the atherogenic mechanism and the study of such an enzyme therefore should have substantial merit. Characterization of LCAT has long been precluded by the lack of homogeneous preparations. 1) We will purify LCAT and study its physical, chemical and enzymatic properties. Purification will involve (a) preparative ultracentrifugation, (b) affinity chromatography and (c) column chromatography. (2) Physical properties (molecular weight and subunit structure) will be studies by (a) analytical ultracentrifugation (b) gel filtration in the presence and absence of diaggregating agents. (3) Study of the chemical properties will include the following methods (a) amino acid analysis (b) N and C terminal determinations (c) tryptic fingerprinting. (4) Enzymatic properties such as (a) substrate specificity (b) activators and inhibitors (c) involvement of SH groups in catalytic activity will also be investigated.